Absorbent structure designed for absorbing body fluids

ABSTRACT

An absorbent structure designed for use in absorbent pads such as diapers is disclosed. According to a first aspect of the invention, absorbent pads are provided which comprise at least two layers of cellulosic fibers (fluff) of different densities or average pore size ranges. When the absorbent structure is incorporated in a diaper, the lower density element extends to the full length and width dimensions of the normal absorbent component of the diaper. The higher density element lies beneath at least a portion of the lower density element. According to various embodiments of the present invention, it can run as a strip from the target zone to the back waist of a diaper, it can run along a central portion extending the whole length of the diaper, or it can underlie the entirety of the lower density element. The higher density element (or the element having a lower average pore size) wiscks waste fluid from a lower central portion of the diaper, and thereby transports the fluid upwardly and also transfers it to the lower density element (or the element of greater average pore size). According to a second aspect of the invention, which may be incorporated with the first aspect just described, a portion of superabsorbent material is placed beneath the higher density layer, (or layer of lower average pore size), such that an additional reservoir for absorbing waste fluid is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of copending, commonly assigned U.S. Pat.Application Ser. No. 886,780, filed July 17, 1986, by Leo J. Bernardin,now U.S. Pat. No. 4,699,619, which is a continuation-in-part ofcopending, commonly assigned U.S. Pat. Application Ser. No. 656,384,filed Aug. 31, 1984, by Leo J. Bernardin, now abandoned, which is acontinuation-in-part of copending, commonly assigned U.S. Pat.Application Ser. No. 599,882, filed Apr. 13, 1984, by Leo J. Bernardin,now abandoned.

BACKGROUND OF THE INVENTION

Disposable diapers generally include an inner, liquid permeable linerthat lies closest to the wearer's skin. An outer, liquid impermeablecover is disposed at the outside of the diaper, furthest away from thewearer. In between the liner and the cover is a fibrous liquid absorbentmaterial or batt.

In general, the most economical liquid absorbent material for use indisposable diapers is cellulosic fibers such as comminuted wood pulp,commonly known in the art as "fluff pulp," or simply "fluff." Batts madefrom these fibers have a low density and a high capacity for absorbingfluids, but their wicking ability is very poor. Thus, the low centralcrotch portion of the diaper as worn tends to become very wet when wastefluids are released thereupon, while the fluff included in the higherportions of the diaper (as worn) can remain dry with its absorbentcapacity unused.

Several problems are posed by this. First, it is uneconomical to line adiaper with fluff, the majority of which will be wasted. Further, therecan be leakage from the liquid saturated central portion of the diaper.Still further, this concentration of liquid localized in one region ofthe diaper can cause discomfort to the wearer of the diaper.

One prior art approach noted by Sigl in U.S. Pat. No. 4,213,459 to avoidsome of these problems is to decrease the pore size such that theimproved capillary action of the pores overcomes the force of gravitysufficiently to cause the waste liquid to move upwardly in the diaper.However, it has been noted that the total absorption capacity is reducedby reducing the average pore size. Nevertheless, the use of thisapproach to achieve a better distribution of fluid throughout theavailable area in the diaper has generally been the approach of choicedespite the resulting loss in absorption capacity.

A further step was taken towards the solution of these problems by Sigl(in U.S. Pat. No. 4,213,459, assigned to the assignee of the presentinvention). Sigl discloses providing a localized area of increaseddensity fluff in the batt for wicking waste fluid to a higher portion ofthe diaper. Although this presented an advance in the art, it was notwithout drawbacks. First, the surface of the diaper nearest the wearer'sskin directly beneath the lining becomes soaked. Further, the onlyplaces where liquid could be transferred from the lower density materialto the higher density material were at the borders or edges where thelower density material adjoins the higher density material. Thus, it wassuggested in that patent that the areas around the densified region ofthe batt should remain undensified.

The present invention provides a further advancement in the art, as willbe fully explained below.

SUMMARY OF THE INVENTION

This invention is directed to an improved absorbent structure designedfor use in absorbent pads such as diapers, for absorbing body fluids.

According to a first aspect of this invention, the absorbent pad or battcomprises at least two layers of cellulosic fibers of differentdensities or average pore sizes. In a disposable diaper having a bodyfacing, liquid permeable liner, an outer liquid impermeable cover and aliquid absorbing material therebetween, the improvement of thisinvention is directed to providing as the absorbing material, a firstlayer of cellulosic fiber having a first density or average pore sizeand underlying the permeable lining. A second layer of cellulosic fiberformed substantially separately from the first layer and having a higherdensity or a lower average pore size than the first layer underlies atleast a portion of the first layer.

According to a second aspect of the invention, a superabsorbent materialis inserted adjacent the higher density layer of cellulosic fiber.According to one preferred embodiment, the superabsorbent material issandwiched between two densified layers of cellulosic fiber, both ofwhich are disposed beneath the lower density layer. According to anotherpreferred embodiment, the superabsorbent material underlies the secondlayer of cellulosic fiber.

The first aspect can be used alone or in combination with the secondaspect. Objects and advantages of the invention will best be understoodwith reference to the following detailed description read in connectionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective plan view of a first preferred embodiment of theinvention without the superabsorbent material included therein.

FIG. 2 is a longitudinal cross-sectional view taken along line 2--2 ofFIG. 1.

FIG. 3 is a perspective plan view of a second preferred embodiment ofthe present invention, without the superabsorbent included therein.

FIG. 4 is a longitudinal cross-sectional view taken along line 4--4 ofFIG. 3.

FIG. 5 is a perspective plan view with several cutaway portions of athird preferred embodiment of the present invention without thesuperabsorbent material included therein.

FIG. 6 is a perspective plan view of a fourth preferred embodiment ofthe present invention, including a portion of superabsorbent materialincorporated therein.

FIG. 7 is a longitudinal cross-sectional view taken along line 7--7 ofFIG. 6.

FIG. 8 is a perspective plan view of a fifth preferred embodiment of thepresent invention including a portion of superabsorbent materialincorporated therein.

FIG. 9 is a longitudinal cross-sectional view taken along line 9--9 ofFIG. 8.

FIG. 10 is an absorbency profile for the control diaper tested inExample 1.

FIG. 11 is an absorbency profile for the diaper containing one ply ofcompressed fluff in addition to the regular fluff component as discussedin Example 1.

FIG. 12 is an absorbency profile for the diaper containing two plies ofcompressed fluff in addition to the regular fluff component as discussedin Example 1.

FIG. 13 is the absorbency profile for the diaper containing three pliesof compressed fluff in addition to the regular fluff component asdescribed in Example 1.

FIG. 14 is the absorbency profile for the diaper containing two plies ofcompressed fluff plus a portion of superabsorbent material as describedin Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, the disposable diaper 1 in which thepresent invention is embodied comprises generally a body facing, liquidpermeable liner 2, an outer liquid impermeable cover 3, and a liquidabsorbing material indicated generally as 4 therebetween. The focus ofthis invention is directed to the liquid absorbing material 4. Theliquid absorbing material 4 comprises a first, lower density layer 5 ofcellulosic fibers such as comminuted wood pulp (fluff) which liesbeneath the inner liner 2. A second, higher density layer 6 of flufflies beneath at least a portion of the first lower density layer 5 suchthat waste fluid can be transferred from the lower density layer 5 tothe higher density layer 6. Thus, a lower surface 15 of the lowerdensity layer 5 should be contiguous with at least a portion of an uppersurface 16 of the higher density layer 6. However, a tissue layer can beplaced between the layers 5 and 6 without departing from the scope ofthis invention.

Predominantly softwood pulp fluff can be used for each of the layers 5and 6. Alternatively, a layer of predominantly softwood fluff can beused for the layer 5 and a layer of preponderantly hardwood fluff can beused for the layer 6, in which case it is the average pore size and notnecessarily the density that varies between the layers 5 and 6, as willshortly be explained. For the purposes of this disclosure and theaccompanying claims, "predominantly" is intended to mean at least about80% while "preponderantly" is intended to mean at least about 50%.

In the first preferred embodiment shown in FIGS. 1 and 2, the higherdensity layer 6 is formed as a strip extending from a target area 7,where liquid waste will initially accumulate, to an upper edge 8 towardsthe back of the diaper. The higher density layer 6 draws waste fluidfrom the lower density layer 5 in the target area 7 upwardly towards theedge 8 at the back of the diaper 1. Further, along upper regions 18 ofthe higher density layer 6 towards the upper edge 8 there is,surprisingly, substantial fluid transfer back from the higher densitylayer 6 to the lower density layer 5. It is important to note thatalthough this flowback occurs from the higher density layer 6 to thelower density layer 5, the lower density layer 5 only draws sufficientfluid to satisfy its unsaturated capillary forces proximate to thehigher density layer 6, where there is an overlap in pore sizes suchthat the smallest pores in the lower density layer 5 are smaller thanthe largest pores in the higher density layer 6. The lower density layer5 drains fluid from the higher density layer 6 but does not becomesoaked, with the advantage that the wearer's comfort is maintained whilea significant proportion of the absorption capacity of the lower densitylayer 5 is utilized.

Alternative embodiments according to this aspect of the invention areshown in FIGS. 3 through 5. According to the second preferred embodimentin FIGS. 3 and 4, the higher density layer 6 extends as a strip alongthe entire length of the diaper. According to the third preferredembodiment shown in FIG. 5, the higher density layer 6 is coextensivewith the entire lower density layer 5.

Yet another alternative to each of the embodiments described above isthat of varying the pore size without necessarily varying the density.For example, the present inventor has discovered that the fine fiberdimensions of hardwood fluff can be utilized to advantage bysubstituting at least about 50%, and preferably about 80% to 100%,hardwood fluff fibers of approximately the same density as the lowerdensity softwood fluff fiber layer 5 for the higher density softwoodfluff fiber layer 6. This can be done because the hardwood fluff has asmaller pore size than the softwood fluff material. As a result, ifhardwood fluff fibers are used as a replacement for the higher densitysoftwood fluff fibers, two different pore size distributions will stillbe obtained within the scope of the invention, even if the density ofeach layer is the same. Thus, for example, a two component fluffsandwich comprising a coarse pore structure in the upper layer 5obtained from a predominately softwood fluff pulp and a fine porestructure in the lower layer 6 comprised of a preponderantly hardwoodfluff pulp fluff, densified throughout to one density (preferablyapproximately 0.1 g/cm³) can successfully be used with similar resultsto those obtained according to the embodiments described.

The second aspect of the invention relates to including a portion ofsuperabsorbent material (SAM) 9 as part of the liquid absorbing material4. The term "superabsorbent material" as used herein refers to one ormore hydrocolloid materials capable of absorbing many times its ownweight of water or aqueous fluid. These materials are generally preparedby polymerizing one or more monomers which, if homopolymerized byconventional methods, would form water soluble polymers. To render themwater-insoluble and suitable for the present invention, these polymersor mixture of polymers are typically reacted, frequently with acrosslinking agent, to form crosslinked polymers, thereby introducing alimited water-insolubility while retaining susceptibility to swelling inwater and water-containing fluids. Pseudocrosslinking may also beachieved by chain entanglement of high-molecular weight polymers, thuseffecting water insolubility (see, e.g., U.S. Pat. No. 4,340,706).Typically, these hydrocolloids are salts of polyacrylic acid andvariations thereof, such as methacrylic acid. Commercially they areavailable under the trademarks WATER LOCK J-500 from Grain ProcessingCo.; ABASORB 720 from Arakawa Chemical, (U.S.A.) Inc.; and Aqualic-CAfrom Mitsui Co. Alternative superabsorbents also may include hydrophilicpolymer grafts onto starch or cellulose backbones and crosslinkedcarboxylated celluloses.

In FIGS. 6 and 7 the fourth preferred embodiment is shown wherein thesuperabsorbent material 9 is sandwiched between a first higher densitycomponent 10 and a second higher density component 11. As can be seenfrom reference to FIGS. 6 and 7, the superabsorbent material 9 islocated more towards one of said end edges than the other. These firstand second higher density components 10, 11 are disposed in anequivalent position to the higher density layer 6 in the first threeembodiments discussed above. However, it is generally preferred that thesuperabsorbent material 9 is localized in an area 17 slightly above aregion midway between the target area 7 and the upper edge 8. Thus, thesuperabsorbent material 9 provides an additional reservoir for liquidwaste that has been drawn up the diaper by the higher density layer 6.

An important advantage of this aspect of the invention is that itassists in fluid distribution along the length of the superabsorbentmaterial. Generally, superabsorbents 9 disposed in a fiber matrixenhance fluid distribution in the direction corresponding to thethickness of the liquid absorbing material 4. However, due to theswelling nature of the superabsorbent material 9, wicking in the planeof the fiber matrix bearing the superabsorbent 9 is severely hindered.According to this aspect of the present invention, wicking across theentire length of the superabsorbent material 9 is accomplished bysandwiching the superabsorbent material 9 between the first high densitycomponent 10 and the second high density component 11. Thus, the liquidwaste is brought into contact across an upper surface 12 and a lowersurface 13 of the superabsorbent material 9 rather than across a bottomedge 14 thereof. This is an improvement over border-to-border transferof liquid waste, in much the same way as the first aspect of theinvention, described in connection with FIGS. 1-5, provides theimprovement of surface to surface contact between the lower densitylayer 5 and the higher density layer 6.

FIGS. 8 and 9 show an alternative embodiment wherein the superabsorbentmaterial 9 is disposed adjacent the outer, liquid impermeable cover 3,and then covered with the single higher density layer 6 which is in turncovered with the lower density layer 5.

Of course, still further alternative embodiments are provided by using asoftwood fluff pulp in the layer 5 and a hardwood fluff pulp in thelayers 6 (FIGS. 8 and 9), or 10 and 11 (FIGS. 6 and 7), with uniformdensification of all layers to about 0.1 g/cm³, or in the range of about0.1 g/cm³ to about 0.15 g/cm³. Yet further variations within the scopeof this invention include using a softwood fluff pulp of a first densityin combination with a hard wood fluff pulp of a second density, whereinthe second density is preferably higher than the first density.

The term "density" as used herein refers to the density of the compositestructure of the lower density layer 5 or the higher density layer 6,and not the actual fiber density. (The actual fiber density is about 1.5g/cm³.) The density of the lower density layer 5 should be in the rangeof about 0.03 to about 0.14 g/cm³, with the preferred range, in light ofconsiderations of integrity and bulk, at about 0.07 to about 0.11 g/cm³.The higher density layer 6 should have a density in the range of about0.14 to about 0.3 g/cm³, and preferably in the range of about 0.16 toabout 0.22 g/cm³ for providing the desired capillarity without excessivestiffness. The density for these preferred ranges is a density measuredunder a load of 0.2 psi. An important consideration in selectingsuitable ranges for the higher density fluff and lower density fluff isthat although there should be two distinct pore size distributions, onefor each of the respective layers of fluff, these pore sizedistributions should overlap because this will permit flowback from thehigh density layer back to the low density layer.

The densified layer 6 may be a multiple ply layer rather than merely asingle ply layer, and the ply may increase in density across the widthor length of the layer 6 within the specified range or can be of uniformdensity. The basis weight of the lower density layer 5 should be in therange of about 150 to about 500 g/m² to provide the desired aestheticappearance and comfort of the diaper. The basis weight of the higherdensity layer 6 should be in the range of about 150 to 1250 g/m².

It is preferred that the higher density layer 6 be substantiallyseparately rather than integrally formed with the lower density layer 5,to achieve better moisture distribution therethrough. It should benoted, however, that the layers 5 and 6 are "substantially separately"formed even when formed in the same machine, one on top of the other,with some admixing at the interface. By the "substantial separate"terminology, it is intended only that two substantially discrete layersare formed, rather than a substantially single layer having a "skinned"surface, such as described in U.S. Pat. Nos. 2,952,260, 2,955,641,3,017,304, 3,060,936, and 3,494,362 to Burgeni. Thus, densificationshould be achieved uniformly through the higher density layer 6, andcare should be taken to avoid substantial skinning, where such skinningis as described in the Burgeni patents or in the Aberson U.S. Pat. No.4,103,062. Skinning produces undue stiffness in the fluff and results inlow absorbent capacity in the bonded (skinned) layer. Skinning can beprevented by avoiding excess moisture and by maintaining both platens atapproximately the same temperature to avoid condensation through thethickness of the material. The present invention avoids thedensification of the fluff to the point of forming a skin layer so thatthe densified fluff layer 6 retains at least about 80% void volume,resulting in significant absorbent capacity.

The layer 6 is densified in a heated press, such as can be obtained byDake, Grand Haven, Michigan, as a "Dake Laboratory Press", Model No.44-148 which includes heated platen operated under suitable conditionsof temperature and pressure as known in the art to achieve the desireddensity. Alternatively, a heated calender nip is suitable for use in thedensification process, as known in the art.

Comminuted wood pulp (fluff) is preferred for this invention, but othercellulose fibers such as cotton linters can be used. The preferred fluffis southern pine kraft wood pulp (i.e., made according to the sulfateprocess commonly known in the art) which has been bleached, such as canbe purchased from ITT Rayoneer or International Paper Company. Asuitable hardwood fluff pulp is southern hardwood kraft obtainable fromWeyerhauser as "New Bern 309".

The superabsorbent material can be obtained from the Grain ProcessingCompany of Muscatine, Iowa as their product Water Lock™ J-500 or A-100.Suitable inner liners 2 and outer covers 3 are easily obtained and wellknown in the art. Combined with these alternative absorbents is a soft,compressible fluid transfer portion or layer comprised of syntheticfibers such as polyester/polypropylene. This transfer layer isintegrally bonded to a top liner at spaced apart sites to providedensified zones for fluid transfer. Such a web is described in U.S. Pat.No. 4,397,644. The top liner is a pattern bonded spunbonded web also ofsynthetic fibers such as polypropylene, polyester and the like. Thecombined basis weight of the liner/transfer layer should be in the rangeof about 30-100 g/m² with the liner being above 10-15 g/m² and thetransfer layer about 20-90 g/m². It must have a wetting finishthroughout.

The transfer layer is typically a carded web of polyester/polypropylenefiber containing from 0 to 100% polyester fiber, preferably 25-50%polyester staple, containing sufficient surfactant finish to bewettable. Two-inch polyester staple, Type T808, a hollow fiber, at 5.5denier from E. I. duPont Co. with a wettable finish is preferredalthough other wettable polyester staple fibers would work.Polypropylene Type T-123 from Hercules Inc. has a wettable finish withthe 3-denier, 1βinch staple preferred. The web is only bonded togetherthrough the spaced apart bonds which attach it to the top liner, whichare typically achieved through thermal or ultrasonic bonding.

The layer may also be formed by other nonwoven processes such asspunbonding wherein the filaments are spun and collected on a wirescreen without bonding and then bonded to the prebonded liner asdescribed above. It must have a wetting finish as is achieved bytreating with 0.2-0.5% Triton X-102 from Rohm and Haas Co.

Of course, many changes and modifications can be made to the preferredembodiments described herein without departing from the spirit of theinvention. For example, the densified fluff layer 6 could also be usedwith other absorbents such as coformed (meltblown polymer fiberscombined with wood pulp and/or staple fibers) webs, carded webs,air-formed (staple wood pulp blends) webs and the like, at a lowerdensity. Further, the absorbent structures described herein haveapplications to embodiments other than diapers, such as to sanitarynapkins or other hygienic media. Therefore, it is intended that thepreferred embodiments described in this specification be regarded asillustrative rather than limiting and it is intended that the claims,including all equivalents, define the scope of the invention.

The advantages resulting from this invention will be clearly understoodwith respect to the following examples and test data.

EXAMPLE I

The high density fluff was prepared by running the fluff into a hotcalender nip (steel on hard rubber) at 250° F. at a speed of 10 ft/min.The nip pressure was sufficient to give a measured density on standingovernight of 0.15 g/cm³. Alternatively the fluff could have been pressedin a blotter lined platen press heated to about 250° F. for about 5seconds, with sufficient pressure to achieve the desired fluff density.

The lower density fluff was prepared by passing it through a calendernip at 250° F. at a speed of 10ft/min and sufficient pressure to achievea measured density on standing overnight of 0.1 g/cm³.

In both cases the fluff was prepared from southern pine bleached kraftwood pulp, and was cut to dimensions such that the weight conditions setforth in Table I below were met. The combined low density fluff and highdensity fluff weight are given in the first column of Table I. The totalabsorbent weight was maintained at about 37 grams for each diaper,except for the one containing superabsorbent. The diaper withsuperabsorbent located as shown in FIG. 8 contained an additional 7grams of superabsorbent composite consisting of about 3 grams ofparriculate superabsorbent (Water Lock™ A-100 from Grain Processing Co.,Muscatine, Iowa); about 3 grams of polyolefin; and about 1 gram ofporous nonwoven wrap. The composite was made in accordance with theteaching of Mazurak and Fries, as set forth in U.S. Pat. No. 4,381,782.The diaper construction was completed by sandwiching the absorbentcomposite between a porous spunbonded polypropylene liner (23 g/yd²) anda polyethylene film outer cover, sealed together with adhesive at thediaper perimeter. Five types of diapers were constructed in thisfashion, corresponding to the diaper description of Table I.

Each of the diapers was then subjected to a 15 minute vertical winkingtest with synthetic urine, followed by 15 minutes of equilibration outof contact with the urine bath. The results of this testing are shown inTable I and FIGS. 10-14.

In this test the preweighed diapers were mounted on a lucite plate(3/8"×13"×14") with the impervious poly backing adjacent the platesurface. The diaper was symmetrically wrapped around the edge of theplate with the longitudinal dimension of the diaper parallel to the longdimension of the plate. The waist line edges of the diaper were mountedon the plate with masking tape, the elastic leg bands being cut in 2 or3 places to facilitate the mounting.

The plate was suspended vertically over a fluid bath contained in aglass tray with the longitudinal dimension of the diaper perpendicularto the fluid surface. The fluid was then brought into contact with thelower edge of the diaper so that the diaper edge was slightly immersed,and was maintained in this position for 15 minutes. The diaper was thenremoved from fluid contact and allowed to hang in the same verticalorientation for 15 minutes equilibration. The fluid used for the diapersin Table I was a synthetic urine composed of by weight 1% urea, 0.01%Pluronic 10R8 surfactant to bring the surface tension to about 56dynes/cm, and 0.04% sodium azide preservative.

After equiliberation the diapers were removed from the plate and weighedand the fluid pick up noted. The results are summarized in Table I andrepresent the average of seven measurements on each diaper design.

After weighing, the diapers were said horizontally on a 3"×15" cuttingdie which was segmented into 9, 1.7-inch zones with cutting edges acrossthe die width as well as the outer perimeter. Several quick blows with awooden mallet divided the 3-inch wide absorbent strip along thelongitudinal axis of the diaper into 9 segments. The absorbent componentin each segment was weighed, oven dried and reweighed and the fluid pickup determined on a gram per gram or fiber basis (corrected for depositedsolids from the fluid).

In the case of the diaper with the superabsorbent pouch, the syntheticurine composition was adjusted to more closely simulate the electrolytecomposition of baby urine, namely: 0.31 g. CaH₄ (PO₄)₂ H₂ O, 0.68 g. KH₂ PO₄, 0.48 g. MgSO₄ 7H₂ O, 1.33 g. K₂ SO₄, 1.24 g. Na₃ PO₄ 12H₂ O, 4.4g. NaCl, 3.16 g.KCl, 0.4 g. NaN₃, 8.56 urea and 0.1 g. Pluronic 10R8 perliter, using distilled water as the solvent. The components were addedto 900 mls of distilled water in the order given and each dissolvedbefore the next component was added, and finally diluted to 1 liter.

The dotted curve in FIG. 10 represents the equilibrium capacity for thelow density fluff at the indicated hydrostatic tensions as measured in acapillary tension cell. It is apparent that only at short wickingdistances (4.5 cm) does the fluff reach its ultimate capacity. At thegreater wicking heights (9-18 cms) it is well below its capacity.

FIGS. 11-13 demonstrate the ability of the densified fluff layer to pullthe fluid up against the hydrostatic heads indicated. Comparison of thefluid level in the low density fluff in the back of the diaper with thatin the same material in front at the corresponding head is a measure ofthe effectiveness of the high density fluff to improve the utilizationof the low density material (e.g. 2.8 g/g back vs. 0.1 g/g front at 18cms head, FIG. 13).

FIG. 14 shows the effectiveness of the densified layers in carrying andtransferring fluid to the superabsorbent. The levels of fluid pickup inthe superabsorbents are 12 and 22 g/g respectively at the 9 and 13.5 cmheads. Comparing this to FIG. 12 illustrates the effectiveness of thesuperabsorbent in diverting fluid from the fluff components at thehighest heads.

It is apparent from these results that fluid distribution is greatlyenhanced up the back of the diaper in the designs with densified flufflayers. The contrast with the fluid distribution in the front of thediaper, where densified fluff layer is absent, is very evident from thefigures. Of course, better fluid distribution in the front as well asthe back of the diapers could be obtained when the second and thirdpreferred embodiments are followed. However, the first preferredembodiment was followed for the purposes of these tests because thediaper front tends to become fairly thoroughly wetted when the releaseof waste fluid occurs when the child wearing the diaper is lying on itsstomach, for example when it is sleeping.

Moreover, the results demonstrate that the fluid distribution in the lowdensity fluff areas is significant. This demonstrates the unexpectedsynergism obtainable in these designs.

In the instance of the superabsorbent diaper, which was made inaccordance with the fourth embodiment set forth in this specification,the superabsorbent material drained substantial fluid from the higherregions of the diaper. However, these regions toward the upper edge 8 ofthe diaper provide surplus absorbing capacity if needed.

                                      TABLE I                                     __________________________________________________________________________    Whole Diaper Vertical                                                         Wicking Test*                                                                                          Amt. of                                                                             Fluid Ab-                                                 Absorbent                                                                            Weight of                                                                            Fluid sorbed Over                                               Weight Comp. Fluff                                                                          Absorbed                                                                            Control                                        Diaper Description                                                                       g.     g.     g.    %                                              __________________________________________________________________________    Control Diaper                                                                           36      0     140   --                                             Diaper containing 1                                                                      38      7     171   22                                             ply of compressed                                                             fluff                                                                         Diaper with 2 plies                                                                      37     14     191   37                                             of compressed fluff                                                           Diaper with 3 plies                                                                      35     21     203   45                                             of compressed fluff                                                           Diaper with 2 plies of                                                                    43**  14     215   54                                             compressed fluff                                                              & SAM                                                                         __________________________________________________________________________      *Absorption time: 15 minutes followed by 15 minutes equilibration time.       **Contains 7 g. of assembly containing 3 g. SAM.                        

EXAMPLE II

The present inventor has discovered that similar functionality to thehigher density fluff can be achieved with finer fiber, hardwood pulpfluff but at normal density (0.1 g/cm³). This discovery was madefollowing the observation of a remarkable similarity in the pore volumedistribution of 0.1 g/cm³ density hardwood fluff and that of 0.2 g/cm³softwood fluff. The following tables compare the vertical wickingproperties of softwood and hardwood fluff.

The vertical wicking test was somewhat similar to that conducted withwhole diapers described above. This test was conducted on 3"×15" stripsof the absorbent material mounted on a 3/8"×5"×14" lucite plate in thesame orientation and manner described for the diapers. A nylon meshscreen was used to support the fluff strip on the lucite (acrylic)plate. The plate and strip were then suspended from weight-sensing meanssuch as a load cell in an Instron tensile tester. The synthetic urine(in these cases the second or salt-containing formulation) was then putin contact with the lower edge of the absorbent strip and the amount offluid absorbed as a function of time is recorded at several timeintervals for the 15 minute duration of the test. The amount absorbedwas as calculated as the grams per unit basis weight of 1 gm/cm² perunit width (1 inch). The vertical wicking capacity is defined as theamount absorbed in the defined units at the end of the 15 minute test.The reported rate was that measured from the plot of amountabsorbed/unit basis weight/unit width versus time^(1/2) over the timeinterval of 7-21 seconds. The test was replicated six times and theresults reported are averages of these six measurements.

Fluid distribution in this test was measured by die cutting the sampleimmediately following the completion of the test, the same mannerdescribed for the whole diapers. In this case zones corresponding to thesame vertical heights were combined to give the results shown in TableIII.

As may be concluded from the results in Table II, the vertical wickingcapacity of 0.1 g/cm³ density hardwood fluff corresponds closely withthat of 0.2 /cm³ density softwood fluff. The initial wicking rate of thehardwood fluff falls between the two softwood fluffs. The fluiddistribution pattern in Table III also show the similarity of thehardwood fluff and the 0.2 g/cm³ density softwood fluff. These resultsimply a slightly larger average pore size for the former than thelatter.

Hence, a combination of softwood and hardwood fluff at 0.1 g/cm³ densityshould have similar fluid distribution properties to a two-densitysandwich of softwood fluff, as previously described in this disclosure.

                  TABLE II                                                        ______________________________________                                        VERTICAL WICKING CAPACITY/RATE                                                (Showing similarity in capillarity of softwood kraft at                       0.2 g/cm.sup.3 and hardwood kraft at 0.1 g/cm.sup.3 density)                                        *Vertical  Wicking Rate                                              Density  Capacity   G/UBW/UW/                                    Material     (g/cm.sup.3)                                                                           G/UBW/UW   SEC1/2                                       ______________________________________                                        Softwood Kraft Fluff                                                                       0.1      499        48                                           Softwood Kraft Fluff                                                                       0.2      573        36                                           Hardwood Kraft Fluff                                                                       0.1      580        42                                           ______________________________________                                         *Units are grams per unit basis weight (1 g/cm.sup.2) per unit width (1       inch).                                                                   

                                      TABLE III                                   __________________________________________________________________________    FLUID DISTRIBUTION IN VERTICAL WICKING                                                      **Specific Absorbency (g/g) at Following Tensions                             0 cm 4.5 cm                                                                             9 cm 13.5 cm                                                                            18 cm                                       __________________________________________________________________________    Softwood Kraft (0.1 g/cm.sup.3)                                                             9.0  9.0  6.4  3.0  0.7                                         Softwood Kraft (0.2 g/cm.sup.3)                                                             6.0  6.1  5.8  5.0  4.7                                         Hardwood Kraft (0.1 g/cm.sup.3)                                                             8.2  8.1  7.2  5.3  3.3                                         __________________________________________________________________________     **Tensions are measured as vertical distance from fluid source.          

EXAMPLE III

This example is directed to a double fluff layer of softwood fluff overhardwood fluff. The hardwood fluff was a bleached kraft, South AmericanEucalyptus hardwood pulp. The two-ply fluff contained equal weights ofthe softwood and hardwood pulp layers. After densifying the layers to0.1 g/cm³ density, the vertical wicking properties (same test procedureas for example II() were as follows:

                  TABLE IV                                                        ______________________________________                                        VERTICAL WICKING PROPERTIES OF                                                SOFTWOOD/HARDWOOD LAYERED BATT                                                                  ***Specific Absorbency                                      ***Vertical                                                                           ***Wicking                                                                              At Following Tensions                                       Capacity                                                                              Rate      0 cm   4.5 cm                                                                              9 cm 13.5 cm                                                                              18 cm                              ______________________________________                                        544     46        7.8    7.8   6.1  4.3    3.0                                ______________________________________                                         ***Same units as in Tables II, III.                                      

Comparing the distribution results for the double-layered fluff in TableIV with those for softwood kraft (Table III), it is apparent that thepresence of the Eucalyptus layer greatly improves fluid wicking againstthe upper two hydrostatic heads (i.e. in the 13.5 cm and 18 cm zones).This is particularly impressive since the Eucalyptus only represents 50%of the two-ply structure.

EXAMPLE IV

A 40 g/m² web of meltblown polypropylene containing 40 percent by weightof superabsorbent (Waterlock J-500 from Grain Processing Company) wasprepared. The superabsorbent was intermixed with the fine meltblownfibers (surface area about 1 m² /g) containing enough surfactant (e.g.,about 0.3-0.5 percent Triton X-102 from Rohm & Haas Company) to assurewetting of the fiber matrix, the composite being densified to 0.1 g/cm³during the manufacturing process.

When this web was subjected to the vertical wicking procedure describedin Example II, the results presented in Table 5 were obtained. Fromthese results it is apparent that the fluid wicks vertically 4.5 cms butthen essentially stops, presumably because the swelling particles haverestricted the fluid flow in the web by plugging the pores.

When this same web was placed beneath a softwood fluff layer, having adensity of 0.2 g/cm³, in a diaper construction, and the whole diaper wassubjected to the vertical wicking test (described in Example I), theeffect of the fluff layer as a fluid distributor for the superabsorbentwas apparent. In Table 6, the fluid distribution is shown for the totalabsorbent composite, the fluff layer itself, and for themeltblown/superabsorbent web.

Comparing the vertical distribution for the meltblown/superabsorbent webfrom the diaper (Table 6) with the data in Table 5, it is apparent thatthe densified fluff acts to wick the fluid higher and improves thedistribution to the superabsorbent (compare at 9 cm height, 11.6 g/g vs.1.1 g/g; and at 13.5 cm height, 2.7 g/g vs. 0.1 g/g). Note that bothtests were similarly run for 15 minutes. Furthermore, if the tests wereflowed to proceed increase, whereas wicking in the superabsorbent webalone further, the height wicked in the diaper would continue toincrease, whereas wicking in the superabsorbent web alone wouldvirtually stop.

                                      TABLE 5                                     __________________________________________________________________________    VERTICAL WICKING TEST                                                         MELTBLOWN/SUPERABSORBENT WEB                                                           Basis    ***Specific Absorbency                                               Weight                                                                            Density                                                                            at Following Tensions*                                      Description                                                                            g/m.sup.2                                                                         g/cm.sup.3                                                                         0.0 cm                                                                            4.5 cm                                                                            9.0 cm                                                                            13.5 cm                                                                            18.0 cm                                    __________________________________________________________________________    Meltblown Web                                                                           93 0.1   7.9                                                                               7.7                                                                              2.4 0.0  0.0                                        with No Super-                                                                absorbent                                                                     Meltblown Web                                                                          139 0.1  23.0                                                                              20.5                                                                              1.1 0.1  0.0                                        with 40% Super-                                                               absorbent                                                                     __________________________________________________________________________     *Hydrostatic Tension in cm. of water.                                         ***Same units as in Tables II and III.                                   

                                      TABLE 6                                     __________________________________________________________________________    VERTICAL WICKING TEST                                                         ON DIAPER CONTAINING                                                          MELTBLOWN/SUPERABSORBENT WEB                                                                           ***Specific Absorbency                                         Dry Absorbent                                                                         Absorbent                                                                            at Following Tensions*                               Diaper Absorbent                                                                        Weight (g.)                                                                           Component                                                                            0.0 cm                                                                            4.5 cm                                                                            9.0 cm                                                                            13.5 cm                                                                            18.0 cm                             __________________________________________________________________________    Fluff densified                                                                         42.7    Fluff  7.5  7.2                                                                              4.5 2.5  1.5                                 to 0.2 g/cm.sup.3 over                                                                          MB/SAM**                                                                             18.8                                                                              19.7                                                                              11.6                                                                              2.7  1.0                                 coextensive layer Combined                                                                             9.9 10.1                                                                              6.4 2.6  1.5                                 of Meltblown Web  Absorbent                                                   with 40%                                                                      Superabsorbent                                                                (basis weight 139                                                             g/m.sup.2)                                                                    __________________________________________________________________________     *Hydrostatic Tension, cms of water                                            **Meltblown Web/Superabsorbent                                                ***Same units as in Tables II and III.                                   

I claim:
 1. A disposable, absorbent structure suitable for use in anabsorbent article, said absorbent structure having end edges at theopposite ends of the length of the absorbent structure, said absorbentstructure comprising:(A) an upper layer having a density in the range ofabout 0.03 to about 0.4 gram per cubic centimeter and consistingessentially of fiber material, which in batt form has a capacity forabsorbing fluids; and (B) a lower layer in direct fluid communicationwith the upper layer and having a density in the range of about 0.14 toabout 0.3 gram per cubic centimeter and comprising preponderantly amixture of fiber material, which in batt form has a capacity forabsorbing fluids, and superabsorbent material; said lower layer furtherbeing positioned relative to said upper layer in a manner such thatsuperabsorbent material in said lower layer is located more towards oneof said end edges than the other.
 2. A structure in accordance withclaim 1 wherein the weight ratio of said superabsorbent material to saidlower layer is at least about 3:21.
 3. A structure in accordance withclaim 1 wherein the weight ratio of said superabsorbent material to saidlower layer is int he range of about 3:21 to about 40:100.
 4. Astructure in accordance with claim 1 wherein said lower layer ismultilayered and comprises:a first layer having a density of from about0.14 to about 0.3 gram per cubic centimeter and consisting essentiallyof fiber material, which in batt form has a capacity for absorbingfluids, a third layer having a density of from about 0.14 to 0.3 gramper cubic centimeter and consisting essentially of fiber material, whichin batt form has a capacity for absorbing fluids, and a second layerdisposed between said first layer and said third layer and consistingessentially of a substantially uniform combination of fiber material,which in batt form has a capacity for absorbing fluids, and particles ofsuperabsorbent material, wherein the weight ratio of said particles ofsuperabsorbent material to said second layer is in the range of about3:21 to about 40:100; said lower layer having a top surface area whichis less than that of said upper layer, and said lower layer furtherbeing positioned relative to said upper layer in a manner such thatabout all of the superabsorbent material in said lower layer is foundwithin one half of the length of said absorbent structure which includesone of said end edges.
 5. A structure in accordance with claim 4 whereinsaid lower layer has a top surface area which is about 0.25 times thatof said upper layer.
 6. A structure in accordance with claim 1 whereinsaid upper layer consists essentially of cellulose fiber material andsaid fiber material of said lower layer is at least one of a cellulosefiber material and a polypropylene fiber material, said polypropylenefiber material having a surfactant thereon.
 7. A structure in accordancewith claim 6 wherein each said cellulose fiber material is selected fromthe group consisting of soft wood fluff, hard wood fluff, and mixturesthereof.
 8. A structure in accordance with claim 1 wherein saidabsorbent structure comprises one or more non-woven webs selected from agroup consisting of coformed webs, carded webs, melt-blown webs, andair-formed webs.
 9. A structure in accordance with claim 6 wherein saidupper layer is a non-woven web selected from a group consisting ofcoformed webs, carded webs, melt-blown webs, and air-formed webs.
 10. Astructure in accordance with claim 6 wherein said upper layer has alarger average pore size than said lower layer.
 11. A structure inaccordance with claim 10 wherein said fiber material of said upper layeris cellulose fiber material.
 12. A structure in accordance with claim 11wherein said cellulose fiber material is selected from a groupconsisting of soft wood fluff, hard wood fluff, and mixtures thereof.13. A structure in accordance with claim 1 wherein said upper layer hasa lower average basis weight than said lower layer.
 14. A structure inaccordance with claim 1 wherein said lower layer has a top surface areawhich is less than that of said upper layer.
 15. A structure inaccordance with claim 14 wherein said lower layer is in the form of astrip extending only from a target area, where liquid waste initiallyaccumulates, to one of said end edges of said structure.
 16. A structurein accordance with claim 14 wherein said lower layer is in the form of astrip which extends along at least substantially the entire length ofthe structure.
 17. A disposable, absorbent structure suitable for use inan absorbent article, said absorbent structure having first and secondend edges located at opposite ends of the length of said absorbentstructure, said absorbent structure comprising:(A) an upper layer havinga density in the range of about 0.03 to about 0.14 gram per cubiccentimeter and consisting essentially of cellulosic fiber material; and(B) a lower layer in direct fluid communication with the upper layer andhaving a density in the range of about 0.14 to about 0.3 gram per cubiccentimeter and comprising preponderantly superabsorbent material and atleast one of a cellulosic fiber material and a polypropylene fibermaterial, said polypropylene fiber material having a surfactant thereon;said lower layer further being positioned relative to said upper layerin a manner such that the superabsorbent material in said lower layer ispredominantly found within one-half of the length of said absorbentstructure, which includes one of said end edges.
 18. A structure inaccordance with claim 17 wherein the lower layer comprises a mixture ofcellulosic fiber material and superabsorbent material.
 19. A structurein accordance with claim 17 wherein the weight ratio of saidsuperabsorbent material to said lower layer is in the range of about3:21 to about 40:100.
 20. A structure in accordance with claim 17wherein the fiber material in said lower layer consists essentially ofcellulose.
 21. A structure in accordance with claim 17 wherein saidlower layer has a top surface area which is less than that of said upperlayer.
 22. A structure in accordance with claim 21 wherein said lowerlayer is in the form of a strip extending from a target area, whereliquid waste initially accumulates, to one of said end edges of saidstructure.
 23. A structure in accordance with claim 22 wherein saidstrip extends along at least substantially the entire length of thestructure.
 24. A disposable, absorbent structure suitable for use in anabsorbent article, said absorbent structure having first and second endedges at opposite ends of the length of said absorbent structure, saidabsorbent structure comprising:(A) an upper layer having a density inthe range of about 0.03 to about 0.14 gram per cubic centimeter andconsisting essentially of fiber material which in batt form has acapacity for absorbing fluids; and (B) a lower layer in direct fluidcommunication with the upper layer and having a density in the range ofabout 0.14 to about 0.3 gram per cubic centimeter and comprising fibermaterial, which in batt form has a capacity for absorbing fluids, andsuperabsorbent material; said lower layer further being positionedrelative to said upper layer in a manner such that at least about all ofthe superabsorbent material in said lower layer is located closer tosaid first end edge than to said second end edge.
 25. A structureaccording to claim 24 wherein the superabsorbent material in said lowerlayer is localized in an area between a target zone, where liquid wasteinitially accumulates, and said first end edge of said structure.
 26. Astructure according to claim 24 wherein the fiber material of said upperlayer consists essentially of cellulosic fibers and wherein the fibermaterial in said lower layer consists essentially of at least one ofcellulosic fiber material and polypropylene fiber material, thepolypropylene fiber material having a surfactant thereon.
 27. Astructure according to claim 24 wherein the superabsorbent material insaid lower layer is mixed with said fiber material of said lower layer.28. A structure according to claim 24 wherein said lower layer has a topsurface area which is less than that of said layer.